The Tokyo Port Management Office confirmed the whale was dead after it was transported to the wharf by two patrol boats.

According to Tokyo Coast Guard Office officials, the corpse was approximately 15-meters-long and is presumed to be a blue whale. It is unclear for how long it had been dead.

Although it is not clear how the whale entered Tokyo Port, the Tokyo Coast Guard Office believes its tail or other body parts may have somehow gotten trapped in the propeller of a large freighter, which carried it there. Source